I've finished easy, normal, hard, and I'm half way through expert so I have quite a bit of experience with the game, and I've found a couple things that bug me.
The Hikers will sometimes get to the third level then immediately climb up right in front, or BEHIND my Queen (and the defenses I have on that level). I find that to be a little unfair. Also there have been times where they climb up but don't stop going up. They just fly off the screen then I have to restart the level because the game is waiting for me to kill that one guy who I can't even see anymore.
Also I wish it would tell you what the next "upgrade" of a monster would be, and its characteristics. I would specifically want this for the Plants. The change from Plant 2 to 3 has a wild decrease of range. I wish I knew that before I upgraded. I would also like to know when the price breaks are during the level ups. A Hive at level (Im' just sitting out numbers) 5 costs 40 bones to upgrade attack from 11 to 12 when a Hive at level 2 costs 90 bones to upgrade attack from 11 to 12. I understand the reasoning behind it, I just wish it would tell us where those breaks are.
One more then I promise I'm done. : ) The exp to upgrade Mudman and Corpes seems a little high to me. The Corpes I understand why the exp has to be high, but the Mudman?! Also does the Corpes exp come from time spent on the map, as opposed to the number of enemies hit?
Anyway, I didn't get any of the lag that people are talking about and I loved the game : D!! A speed up button would be nice though. Great game keep up the good work I hope I see a Monster Castle Defense in the future!